1. Field of the Invention
The present invention relates to an image forming apparatus adopting a technology for detecting the remaining amount of toner.
2. Description of the Related Art
Electrostatic-capacitance detection methods are in widespread use as mechanisms for detecting the remaining amounts of toner in image forming apparatuses using electrophotographic processes. Such electrostatic-capacitance detection methods are disclosed in, for example, Japanese Patent Laid-Open No. 8-44184.
FIGS. 14 and 15 schematically show examples of the configurations of mechanisms for detecting the remaining amount of developer by the electrostatic-capacitance detection method. Toner is used as the developer in the example in FIGS. 14 and 15. A toner container 100 is a container of the developer, and a developing roller 101 is a developer carrier that is provided in the container of the developer and carries and conveys the developer. Referring to FIG. 14, an antenna 104 is a conductor that opposes the developing roller 101 and that is away from the developing roller 101 by a predetermined distance. The antenna 104 and the developing roller 101 form a pair of counter electrodes and an electrostatic capacitance is formed between the antenna 104 and the developing roller 101. The amount of toner between the developing roller 101 and the antenna 104 is decreased as toner 20 in the toner container 100 is consumed to decrease the permittivity between the developing roller 101 and the antenna 104 and also to decrease the electrostatic capacitance therebetween. The difference in voltage between the electrodes is measured to detect a variation in the electrostatic capacitance in order to detect the remaining amount of the toner 20 in the toner container 100. Specifically, when a predetermined alternating (AC) voltage is applied to the developing roller 101 by an AC power supply 105 for detection of the remaining amount of toner, an AC current I1 is generated in accordance with the electrostatic capacitance of an equivalent capacitor 106 formed between the developing roller 101 and the antenna 104. The AC current I1 is in proportion to the product of the frequency of the AC power supply 105 for detection of the remaining amount of toner, the amplitude thereof, and the electrostatic capacitance of the equivalent capacitor 106. The AC current I1 is rectified by a rectifier circuit including diodes 201 and 202, a resistor 203, and a capacitor 204 to convert the AC current I1 into a voltage V1 and the voltage V1 is supplied to the inverting input terminal of an operational amplifier 108. Similarly, when a predetermined AC voltage is applied to a reference capacitor 107 by the AC power supply 105 for detection of the remaining amount of toner, an AC current I2 is generated in accordance with the electrostatic capacitance of the reference capacitor 107. The AC current I2 is rectified by a rectifier circuit including diodes 205 and 206, a resistor 207, and a capacitor 208 to convert the AC current I2 into a reference voltage V2 for detection of the remaining amount of toner and the reference voltage V2 is supplied to the non-inverting input terminal of the operational amplifier 108. The operational amplifier 108, resistors 209 and 210, and a capacitor 211 form an integration circuit, and the difference between the voltage V1 and the reference voltage V2 supplied to the inverting input terminal and the non-inverting input terminal of the operational amplifier 108, respectively, is amplified to be detected as a detection result 109. It is possible to subsequently detect the remaining amount of the toner 20 in the toner container 100 as an analog quantity in the above manner.
Referring to FIG. 15, an RS roller 102 including a dielectric material is in contact with the developing roller 101 to supply the toner to the developing roller 101. The RS roller 102 serves as an electrode member to from a pair of counter electrodes with the developing roller 101. The difference in voltage between the developing roller 101 and the RS roller 102 is measured to detect a variation in the electrostatic capacitance therebetween in order to detect the remaining amount of the toner 20 in the toner container 100.
The AC current I1 is generated in accordance with the electrostatic capacitance of the equivalent capacitor 106 formed between the antenna 104 and the developing roller 101 or between the RS roller 102 and the developing roller 101. The AC current I1 is represented by Equation (1):I1=2πf·Vpp·Ct  (1)where “f” denotes the frequency of an AC voltage 30 for detection of the remaining amount of toner, “Vpp” denotes the amplitude thereof, and “Ct” denotes the electrostatic capacitance of the equivalent capacitor.
Similarly, the AC current I2 generated in accordance with the electrostatic capacitance of the reference capacitor 107 is represented by Equation (2):I2=2πf·Vpp·Cref  (2)where “Cref” denotes the electrostatic capacitance of the reference capacitor.
In general, the electrostatic capacitance Ct of the equivalent capacitor has frequency characteristics different from those of the electrostatic capacitance Cref of the reference capacitor. In addition, the rectifier circuits that rectify the AC current I1 and the AC current I2 to convert the AC current I1 and the AC current I2 into the voltage V1 and the reference voltage V2, respectively, also have frequency characteristics. Accordingly, in order to compare the AC current I1 with the AC current I2, that is, in order to compare the voltage V1 with the reference voltage V2 to perform the satisfactory comparison between the electrostatic capacitance of the equivalent capacitor and that of the reference capacitor, it is necessary to keep the frequency f of the AC voltage 30 for detection of the remaining amount of toner and the amplitude Vpp thereof constant in Equations (1) and (2). Consequently, it is preferred that the amplitude Vpp of the AC voltage 30 for detection of the remaining amount of toner be kept constant and that the AC voltage 30 for detection of the remaining amount of toner have a substantially sine waveform so that the a single frequency that does not contain a harmonic component appears as the frequency f. As a result, the circuitry of the AC power supply 105 for detection of the remaining amount of toner is configured such that the AC voltage 30 for detection of the remaining amount of toner to be output has a substantially sine waveform.
FIG. 16 is a block diagram showing an example of configuration of the AC power supply 105 for detection of the remaining amount of toner. Referring to FIG. 16, a voltage output from a voltage controlling unit 508 is converted into a square-wave voltage by an inverter unit 501. The square-wave voltage is converted into a substantially-sine-wave voltage by a band pass filter 502, and the substantially-sine wave voltage passes through a push-pull amplifier 503 and a high-voltage transformer 504 to generate the AC voltage 30 for detection of the remaining amount of toner having a substantially sine waveform. The voltage controlling unit 508 receives a signal that results from rectification of the AC voltage 30 for detection of the remaining amount of toner by a rectifier unit 505 and that is detected by a voltage detecting unit 506 and a signal supplied from a voltage setting unit 507. The voltage controlling unit 508 supplies a signal that generates the AC voltage 30 for detection of the remaining amount of toner having an amplitude based on the signal supplied from the voltage setting unit 507 to the inverter unit 501 to perform constant voltage control.
FIG. 17 is an exemplary circuit diagram of the AC power supply 105 for detection of the remaining amount of toner. Referring to FIG. 17, a clock signal CLK transmitted from the controller of an image forming apparatus (not shown) is supplied to the gate terminal of a field effect transistor (FET) 601 through a connection terminal 628, and an output voltage supplied from an operational amplifier 624 through a resistor 627 is converted into a square-wave voltage in response to the clock signal CLK. The square-wave voltage is converted into a substantially-sine-wave voltage by a low pass filter including resistors 602 and 603 and capacitors 604 and 605. The substantially-sine-wave voltage is supplied to a push-pull amplifier including resistors 606, 609, 612, and 613, diodes 607 and 608, and transistors 610 and 611. The direct-current (DC) component of the output from the push-pull amplifier is removed by an electrolytic capacitor 614, and the output from the electrolytic capacitor 614 subjected to the DC component removal is converted into the AC voltage 30 for detection of the remaining amount of toner having a substantially sine waveform by a high-voltage transformer 615. The AC voltage 30 for detection of the remaining amount of toner is rectified by capacitors 616 and 619 and diodes 617 and 618 and is supplied to the inverting input terminal of the operational amplifier 624 through resistors 620, 621, 622, and 623 as a detection signal. A voltage setting signal CONT_T transmitted from the controller of the image forming apparatus (not shown) is supplied to the non-inverting input terminal of the operational amplifier 624 through a connection terminal 629. The operational amplifier 624 supplies a signal that generates the AC voltage 30 for detection of the remaining amount of toner having an amplitude based on the voltage setting signal CONT_T to the FET 601 through an integration circuit including a resistor 625 and a capacitor 626 to perform the constant voltage control.
As described above, the AC power supply 105 for detection of the remaining amount of toner uses the band pass filter 502, the push-pull amplifier 503, the high-voltage transformer 504, etc. to generate the AC voltage 30 for detection of the remaining amount of toner having a substantially sine waveform in order to improve the precision of the detection of the remaining amount of toner. However, since it is necessary to provide the multiple circuit components in such a configuration of the power supply, it is difficult to reduce the size of the power supply. Furthermore, the provision of the high-voltage power supply for detection of the remaining amount of toner causes an increase of the cost of the image forming apparatus.